Communication systems, such as cellular communication systems or trunked two-way communication systems, are known to comprise a plurality of communication units and a plurality of base sites. In a cellular communication system, each base site includes one or more base stations and provides communication services to a particular coverage area or cell. Each cell is typically partitioned into a plurality of sectors by using directional antennas at the base site serving the cell to increase the capacity of the cellular system. Each sector of a cell is served by one of the base stations at the base site and is allocated a particular set of channels to use for communications in that sector. Each channel might comprise a receive/transmit frequency pair in a frequency division multiple access (FDMA) communication system, a receive/transmit slot pair in a time division multiple access (TDMA) communication system, or a designated portion of an allocated system bandwidth in a spread spectrum communication system. The channels assigned to each set are determined by the desired channel reuse pattern for the cellular system.
To maximize capacity, without degrading signal quality due to an abundance of co-channel interference and noise, cellular systems typically employ channel reuse patterns of four and seven. In a four cell reuse pattern, all the allocatable channels (i.e., those used for two-way communications) are divided into four sets, one set for each cell. Similarly, in a seven cell reuse pattern, all the allocatable channels are divided into seven sets. However, to maintain a desired signal quality (e.g., a carrier-to-interference plus noise, C/I+N, of 17 dB), each cell in a four cell reuse pattern is typically divided into six 60 degree sectors; whereas, each cell in a seven cell reuse pattern it typically divided into three 120 degree sectors. Once a cell is sectorized, the particular set of channels assigned to that cell is similarly partitioned, such that each sector in the cell is assigned its own subset of the channels allocated to the cell. The channel partitioning within a cell is typically performed to minimize co-channel interference, adjacent channel interference, and noise in accordance with known techniques.
To provide adequate coverage at the cell boundary, each base site typically employs high power transmitters. The power of each transmitter is dependent upon the sectorization of a cell. For example, a higher power transmitter is necessary for a 120 degree sector than for a 60 degree sector in order to maintain a desired signal level along the sector boundary due to the higher directivity of of an antenna having a 60 degree half power beamwidth (HPBW) as compared with an antenna having a 120 degree HPBW.
As is known, cells in a cellular system are typically hexagonal in shape. Thus, each cell has six neighboring cells. The area near the border between any two cells is generally known as the fringe area, or handoff region, of a cell. Therefore, when a communication unit is communicating and enters the fringe area of its host cell, the base site for the host cell might hand off the communication to a base site serving one of the neighboring cells.
As is also known, cellular communication systems use signalling access formats, such as TDMA, FDMA, and code division multiple access (CDMA), to convey communication signals between communication units and their servicing base stations. In a CDMA system, all communication units in the system transmit uplink communication signals to their respective base station in a common reverse channel bandwidth. However, each uplink communication signal is associated with the serving base station by the inclusion of a pseudo-noise (PN) sequence (in direct sequence CDMA) or by a predetermined frequency hopping pattern (in slow or fast frequency hopping CDMA). The PN sequence and the frequency hopping pattern permit the base station to identify which uplink signals are intended for reception by the base station's receiver. In a similar manner, each base station transmits downlink communication signals to the communication units in its coverage area in a common forward channel bandwidth.
The discovery and technical advances associated with the advent of CDMA has permitted cellular system designers to begin using one cell and one sector channel reuse patterns. One cell channel repeat patterns have been attempted in FDMA and TDMA cellular systems; however, such attempts have required that the systems operate at degraded signal quality levels (e.g., at a C/I+N of 13 dB). By contrast, CDMA one cell and one sector repeat patterns generally can maintain acceptable signal quality levels.
In a one sector CDMA reuse configuration, each sector reuses the same channels as its neighboring sectors, as opposed to partitioning the allocatable channels as in other sectorization schemes. However, a problem arises in a CDMA system using a one sector channel reuse pattern when a communication unit is located in the handoff region between two or more sectors. When the communication unit is in the handoff region, it simultaneously receives downlink communication signals from two or more base station transmitters. Consequently, the communication unit is also receiving the noise, co-channel interference, and adjacent channel interference generated in the common forward channel bandwidth by the base stations since each base station is transmitting to its respective sector using a transmitter that provides adequate coverage at the sector boundary. Thus, the communication unit often cannot distinguish the downlink communication signals from the interference and noise. This indistinguishability results in poor signal quality, dropped calls, and limited capacity of the CDMA system due the forward channel degradation at the fringe areas of the cells.
A similar problem arises at the base station due to poor signal-to-noise performance in the common reverse channel bandwidth of a CDMA system when the communication unit is in the fringe area of a cell. The base station receives uplink communication signals from communication units in it's respective sector and from communication units in fringe areas of neighboring sectors. Thus, if the communication unit does not have enough transmit power to overcome the total noise received by the base station, the base station cannot distinguish the communication unit's uplink signal from the noise.
Therefore, a need exists for a method and apparatus for conveying a communication signal between a communication unit and a base site that services a sectorized coverage area. Such a method and apparatus that improves capacity in either the forward channel, the reverse channel, or both by reducing interference and noise in the respective channel would be an improvement over the prior art.